1. Field of the Invention
This invention relates to the field of measuring liquid viscosity and more specifically to the area in which such measurements are made by utilizing a piezoelectric sensor element.
2. Description of the Prior Art
Viscosity measurements utilizing piezoelectric sensors are generally based on the well-known phenomenon that a dissipative or damping force that resists the motion of an energized piezoelectric element lowers its resonant frequency.
Viscosity measurements have been made routinely in the past by using torsional crystals. Such techniques are described in a text by W. P. Mason, entitled "Piezoelectric Crystals and Their Application to Ultrasonics", D. Van Nostrand Company, Princeton, N.J., 1950, pages 339-350. Torsional crystals are described as being formed from ammonium dihydrogen phosphate ADP. The crystals are cut to have a length dimension defined along the "X" axis. A hole is bored along the "X" axis and a cylindrical crystal is formed by turning it about the center of the bore. The crystal is plated with an electrically conducting material continuously over the inner surface and with a strip lengthwise on the outer surface to form the inner and outer electrodes for energizing the crystal. The sensor is shown as being permanently mounted in a liquid container and filled with the liquid to be measured. Due to size limitations in constructing the torsional vibrating crystals, it has been found that the frequency limit is approximately 500 KHz.
The Mason text also describes (page 349) attempts to measure liquid viscosity using thickness-shear mode crystals, like AT cut quartz. However, the measurements were hindered by the tendency of the elements to vibrate in flexural modes. Consequently it was concluded that since it was difficult to distinguish the viscosity controlled shear mode from the flexural modes it was also difficult to unambiguously determine the viscosity of liquid. In addition, flexural oscillations have longitudinal components that generate sound waves in the liquid. These waves can contribute a substantial additional energy dissipation from the motion of the element as such waves radiate into the liquid and interact with the container. The extent of the dissipation depends on the exact position of the element in the container. Therefore, if the element position is changed, the resonant frequencies of the flexural modes change and thereby further obscure the viscosity-controlled thickness-shear mode.
An attempt to measure the viscosity of sucrose and water solutions using thickness-shear mode crystals was described in an article entitle "Frequency of a Quartz Microbalance in Contact With Liquid", by K. K. Kanazawa et al, appearing in ANAL. CHEM., 57, 1770 (1985). That configuration has the disadvantage of a drag being imposed on the element which increases when the pressure differential across the element increases. Thus, the resonant frequency of that structure depends on the depth of its submersion into the liquid.